DAMAGE!!! REVERSED POLARITY!!!

[planovet]

I accidentally hooked up my battery charger to the battery backwards once....but it only blew the fuse, no damage to the charger. That's why a cheap fuse is so valuable.

[SomewhereInND]

My neighbor hooked his BRAND NEW boat up backwards.

Starter relay shorted out, causing starter to start running (backwards)

By the time he disconnected the battery.
Starter was burnt out, along with almost all the wiring
2 Fires started, but were put out.

GPS/Fishfinder was OK

[Miriam C.]

I believe there should be a device to prevent that or build a circuit to detect correct polarity and actuate a power relay to handle the current

Got fuse!

[planovet]

I dont believe a simple fuse can protect sensitive electronics, but it sure does blow after the damage has been done... LOL

That doesn't sound right but I'm not an electrician. Maybe someone with more experience can weigh in...

[Shadow Catcher]

Need a fairly substantial diode (one way valve). A fuse or circuit breaker will only blow when you exceed its amperage (hopefully).

[Larwyn]

The best bet is to simply pay attention to what you are doing. Connect it right and you do not need all the trouble, expense and complication of additional protective circuits which can fail and cause the system not to work even when it is connected correctly. I would rather depend on a piece of simple copper wire for power than any solid state device or relay contact. But we are discussing the electrical system on a camping trailer, not the space shuttle, so if the electrical system fails it should not be the end of the world (or even the end of the camping trip).

[dh]

The best bet is to simply pay attention to what you are doing. Connect it right and you do not need all the trouble, expense and complication of additional protective circuits which can fail and cause the system not to work even when it is connected correctly. I would rather depend on a piece of simple copper wire for power than any solid state device or relay contact. But we are discussing the electrical system on a camping trailer, not the space shuttle, so if the electrical system fails it should not be the end of the world (or even the end of the camping trip).

Well stated!

[artwebb]

The best bet is to simply pay attention to what you are doing. Connect it right and you do not need all the trouble, expense and complication of additional protective circuits which can fail and cause the system not to work even when it is connected correctly. I would rather depend on a piece of simple copper wire for power than any solid state device or relay contact. But we are discussing the electrical system on a camping trailer, not the space shuttle, so if the electrical system fails it should not be the end of the world (or even the end of the camping trip).

You said it, brother! Anyone can make a mistake but this one is so easily avoided Although I'll admit to cooking a fusible link in my Cutlass once by doing this very thing

[Sam I am]

I use a jump starter battery to power my 12 volt interior lighting and receptacles. It has a female cigarette lighter receptacle built into the case. My interior circuits go to a male plug. I just plug it in - can't get it backwards!

[SomewhereInND]

I believe there should be a device to prevent that or build a circuit to detect correct polarity and actuate a power relay to handle the current

Here was the cause of the electrical fire that I described, and why a fuse did not help.

A starter is almost directly wired to the battery, Just one huge fuse and a heavy duty relay between starter and battery. A starter draws a large amount current, at those current levels devices to check for polarity, either don't exist, or are too expensive.

The problem was caused when the starter relay welded shut, making the starter stay on, it took a while for the huge fuse to blow, by that time the starter relay had melted (one of the fire), shorting the ignition system to the starter power supply thru the relay (fire #1), then the ignition system melted (fire #2).

[artwebb]

That's what saved the starter and such on my old Cutlass, no relay and a fusible link with a reasonable capacity. Sometimes newer ain't better.

[Larwyn]

I believe there should be a device to prevent that or build a circuit to detect correct polarity and actuate a power relay to handle the current

Here was the cause of the electrical fire that I described, and why a fuse did not help.

A starter is almost directly wired to the battery, Just one huge fuse and a heavy duty relay between starter and battery. A starter draws a large amount current, at those current levels devices to check for polarity, either don't exist, or are too expensive.

The problem was caused when the starter relay welded shut, making the starter stay on, it took a while for the huge fuse to blow, by that time the starter relay had melted (one of the fire), shorting the ignition system to the starter power supply thru the relay (fire #1), then the ignition system melted (fire #2).

What part did reverse polarity play in the above chain of events?

[Frog]

Michelin now sells "Smart Jumper Cables" with a built in surge protector that automatically adjusts polarity so you can't hook them up wrong. Hook them up, wait a few seconds for the green LED light to indicate the polarity is adjusted and fire away. About $45.00, 8 gauge, 12' set.

A bit pricy for jumpers but for the careless or uninitiated; no fried electrical circuits or sparky light shows from hooking it up wrong.

[SomewhereInND]

What part did reverse polarity play in the above chain of events?

I believe it was what caused the starter relay to origianally fuse itself, for some reason...

[kludge]

A series diode will always have a voltage drop associated, which may or may not be a problem, but it works. The problem is that it will have to be a BIG diode to take 30-50 Amps of current reliably.

Another way to use a diode, without the voltage drop, is to use it as a "crowbar", after the main fuse, with the cathode connected to the fuse and the anode connected to ground. Reverse voltage will blow the fuse. And probably the diode too. So unless you can field replace the fuse and diode this might leave you out of commosion. On the plus side you can get by with a cheaper diode.

Aside from a diode, the simplest reverse voltage protection is a P-channel MOSFET. An N-channel will work too, connected in the ground lead, some people don't like that, but it hardly matters in this case - we're not talking about a high-side switch vs. low-side, we're just trying to block the current.

N-channels will drop less voltage and need less heat sinking, and probably cost less to boot.

Connected like this:

http://archive.electronicdesign.com/fil ... ure_01.gif

And the MOSFET is much faster than a fuse.